Transcoding is quite a daunting task to do...

but what if I already have 100% of my videos encoded using handbrake and know without transcoding they play the video + audio 100% perfectly fine on ALL my devices..

The only thing I really need is burning in the subtitles.

it's basically just "maintain the video stream" and replace the pixels where applicable for the subtitles with bright white pixels.

this should be doable at a rate easy of 100+ Mbps

1) does ffmpeg have such an option: leave video stream 1 intact, PASSTHROUGH and just merge in the white pixels
2) I suppose ffmpeg being extremely feature rich can do this, how/what profile we chose to make mezzmo doing this?

Instead of re-encoding a 100% compatible H.264 into a 100% compatible H.264